The reliability of an offshore pile foundation under extreme environmental loading depends on the uncertainty in the loads acting on the foundation, uncertainty in the geomechanical soil parameters that formed the design basis and the method uncertainty associated with the pile design method used for estimating the pile capacity. To evaluate the required resistance factor for achieving a target reliability level, a reliability-based study of the axial pile capacity for an offshore jacket was conducted using the API and four CPT-based design methods (Fugro, ICP, NGI and UWA methods) and the first-order reliability method (FORM). The study included a statistical evaluation of the soil and load parameters and a quantification of the method uncertainty for each of the five pile design methods. The analyses comprised deterministic and probabilistic analyses of axial capacity for the piles supporting the platform, calculation of the annual probability of failure of the piled foundation and calibration of the required resistance factor for ensuring that the annual failure probability is less than target values of 10/sub -3/, 10/sub -4/ and 10/sub -5/. The results show that the reliability level of piles designed with the different axial pile capacity methods varied significantly, and that the CPT-based methods have lower uncertainty than the API method.
Zhongqiang Liu, Sarah Elkhatib, Farrokh Nadim and Suzanne Lacasse
Keywords: reliability, piles, axial capacity, resistance factors, failure probability
Publisher: DFI